This invention relates to sealing arrangements in general, in particular a sealing arrangement between first and second relatively rotatable bodies of a machine, more particularly a turbo machine.
The development of brush seal technology over the last two decades has principally been in the field of gas turbines. In such applications, the pressure drop across the seal assembly will not usually be more than 25 bar.
However, in steam turbine applications, sealing arrangements will often have to be designed for pressure drops significantly greater than 25 bar. By way of example, in a high pressure cylinder the steam inlet pressure may be of the order of 200 bar and the exhaust pressure may be of the order of 80 bar, for example. In an intermediate pressure cylinder the inlet pressure may be about 80 bar and the exhaust pressure about 5 bar, for example. Thus, in each case, the pressure drop across the balance piston and the pressure drop between the exhaust pressure and the external atmospheric pressure will be considerable.
Various seal arrangements using brush seals have been proposed with the aim of achieving good efficiency and long life. EP-A-0 836 040 discloses a sealing arrangement comprising a series of sealing units 1 (one of which is shown in FIG. 1) between a stationary casing 2 and the rotor 3 of a turbo machine. Each sealing unit 1 includes several brush seals 4 fixed on a carrier 6 mounted on the casing 2 so as to be movable to a limited extent in the radial direction. Fin seals 7 are fixed to the rotor 3.
GB-B-2 301 635 discloses as somewhat similar sealing arrangement (as shown in FIG. 2), in which the carrier 6 has a only a single brush seal 4, which is mounted so as to be capable of limited radial movement relative to the carrier 6. The fin seals 7 are provided on the carrier 6.
It has been found that, in sealing arrangements comprising a series of brush seals, the individual brush seals are prone to failure. It appears that, once one of the brush seals has failed, further failures occur in a cascade fashion because of the increased loading on the remaining seals. It would therefore be desirable to be able to provide a sealing arrangement utilising a brush seal and having a sufficiently long life.